1. Field of the Invention
The invention is related to safety devices and more particularly to an improved safety device for a gear unit driven by a driver mechanism and having a rated breaking point.
2. Description of the Prior Art
One safety device of the type with which the invention is concerned is already known (International Patent Disclosure WO 97/43557) for a gear unit for adjusting flaps on load-bearing surfaces of aircraft. There, the rated breaking point is embodied as a cross-sectional weakening on a shaft. The shaft connects a wheel disk of a first gear wheel to a hub of a second gear wheel. The two gear wheels and the shaft having the rated breaking point form a torque-transmitting gear member, which is disposed between a drive element and power takeoff element of the gear unit. The rated breaking point of the known safety device is intended to disconnect the drive side and the power takeoff side of the gear unit if an overload on the gear unit occurs on the power takeoff side. A silent ratchet in the form of a multiple disk brake that then becomes operative prevents the gear part on the power takeoff side from shifting, on its own. On the drive side, however, the gear unit remains functional, because of the absolute necessity, in such a case, of being capable of driving other gear units by a shaft that connects all the gear units in order to adjust flaps.
A safety device is also known in secondary power takeoff mechanisms of vehicle transmissions (German Utility Model DE 296 12 824 U1), in which a torque-transmitting component is equipped with a rated breaking point. This component is sleevelike, and a cross-sectional weakening, embodied as a notch-type groove, is locate between a component portion that meshes with a drivable intermediate shaft and a power takeoff flange, by which a torque (operating moment) can be transmitted to devices that are screwed on and are to be driven.
In the event of an overload, for example if a device to be driven is blocked, the flange is severed from the driven component portion by an overload moment with release of the torque engagement. The safety device is operative even if, with the drive mechanism switched off, the overload originates at the device on the power takeoff side. A cap engaging the flange prevents the flange, after the rated breaking point is severed, from being lost or from spinning out of control. Although in many ways this safety device can meet the demands made of it, nevertheless in some applications, the nondriven, flanged-on device can present a risk.
The safety device of the invention has the advantage over the prior art that to limit damage, in the event of an overload on the gear unit on the power takeoff side, parts of the gear unit disposed on both the drive side and the power takeoff side of the rated breaking point are held captive. This is attained in that in the event of an overload moment originating on the power takeoff side, after the severing of the rated breaking point that occurs because of the self-locking of the gear unit, the cam member formed onto the hub engages the rib after a fraction of one hub revolution and presses the rib against the wall of the housing recess. At least from frictional engagement, this blocks further rotation of the hub and wheel disk, and as a consequence the device can no longer be moved (blocking moment). The torque, originating on the power takeoff side, is diverted to the housing by the silent ratchet. Blocking the wheel disk in turn protects the drive-side part of the gear unit from the aforementioned torque, so that this part of the gear unit and its drive motor cannot be damaged, and once the destroyed gear member has been replaced with a new one,
The use, for example a back rest adjusting device for a vehicle seat, of the safety device of the invention is of substantial advantage, since in the event of an accident, the blockage of the gear member that becomes operative prevents the seat from shifting relative to the vehicle. The disposition of the safety device of the invention in a back rest adjusting device of the vehicle seat is also advantageous. There, it prevents the back rest from flipping over. This considerably lessens the risk of injury to the person seated on the seat in the event of an accident. Until now the load acting the seat had to be more or less absorbed by a gear unit, dimensioned correspondingly complicatedly (European Patent Disclosures EP 0 359 008 B1 and EP 0 367 096 A2); now, when the safety device is employed, the part on the drive side of the gear unit can be designed in such a way that it absorbs the requisite overload moment for response of the safety device unharmed, and conversely remains free of the very much greater blocking moment. The drive motor and gear unit of a so-called seat adjuster or of a back rest adjuster can therefore be produced more economically and after an accident can be put back into proper condition by replacing the gear member.